1. Field of the Invention
The present invention relates generally to the art of servomechanisms, more especially to servomechanisms enabling the automatic flight control of aircraft, and particularly to the provision of effective and reliable stall current limiting of servo motors driving aircraft control surfaces.
2. Description of the Prior Art
Servo systems of the past have, in many applications, been output torque-limited mechanically or electrically. Mechanical limiter systems have involved slip clutches, but these demonstrate wear, material fatigue, adhesion after long intervals of none-use, and undesired response to lubricants, particulate matter, and the like. Such deficiencies result in unpredictable responses over a wide range of input threshold values.
In electrical limiters for servomechanisms, it has been customary to place a finite resistor in series with the power supply driving the servo motor which resistor, in cooperation with the impedance of the servo motor itself, provides a limit on the maximum current that can be drawn by the servo motor. Experience with this type of series torque limiter has shown its several disadvantages, including variation in the torque limit caused by power supply variations, temperature variation, and changes in the servo motor impedance, and inefficiency caused by the attendant power loss in the series resistor.
In more recent servo systems, such as those of the Rauschelbach patent to be later discussed herein, control of the torque of an automatic pilot servo motor involves feeding the servo motor current driving the loaded surface back to the servo input through a current limiter to limit the maximum current that can be supplied to the motor. However, the patented system limits current and, hence, output torque for both high and low frequency components of the input command signal and is therefore found not to be desirable particularly where boosted control systems are needed, such as in helicopter flight control systems where a series actuator accepting both autopilot and manual commands is to operate into a high power boost surface actuator. Without current limiting, the series actuator may be forced hardover into a full stall for an extended time, possibly permanently damaging it by overheating. With the current limiting system of the prior art, actuator travel speed is undesirably reduced and the servo loop band width is degraded.